Understanding Ambient Air Stability
Ambient air stability refers to the tendency of air parcels to either rise or sink in the atmosphere, which is influenced by the ambient lapse rate. The lapse rate describes how temperature changes with altitude.
What is the Ambient Lapse Rate?
- The ambient lapse rate is the rate at which air temperature decreases with an increase in altitude.
- It is measured in degrees Celsius per kilometer (°C/km).
Types of Lapse Rates:
- Super-adiabatic: Occurs when the lapse rate is greater than the dry adiabatic lapse rate (approximately 9.8°C/km). This condition leads to unstable air, promoting rapid vertical movement.
- Neutral Stability: The lapse rate is equal to the dry adiabatic lapse rate. Air parcels neither rise nor sink significantly, indicating a neutral stability condition.
- Hyper-adiabatic: Similar to super-adiabatic, this also indicates instability, where the temperature decreases even faster than the dry adiabatic rate.
- Sub-adiabatic: This is when the ambient lapse rate is less than the dry adiabatic lapse rate (less than 9.8°C/km). Under these conditions, the air is stable, meaning that any rising air parcel will cool more quickly than the surrounding air, causing it to sink back to its original position.
Why is Option 'D' Correct?
- The correct answer is sub-adiabatic because it indicates stability in the atmosphere.
- Stability occurs when the surrounding air is cooler than a rising air parcel, preventing it from continuing to ascend.
In summary, when the ambient lapse rate is sub-adiabatic, the atmosphere is stable, making option 'D' the correct choice.